Reflector system for determining position

ABSTRACT

The invention relates to a reflector system for positioning a medical instrument or of body parts of a patient or for any determination of position, which is characterized by reflectors ( 3, 4, 5 ) that are configured as transparent retroreflective spheres and may consist of a material with a refractive index of preferably 1.9.

[0001] The invention concerns a reflector system for determiningposition, as set forth in the classifying portion of claim 1.

[0002] The optical navigational systems which are known per se from thestate of the art use either active optical marks or passive marks, fordetermining position. Generally light emitting diodes (LEDs) are used asthe active marks. The passive marks are usually balls with a reflectivecoating or retroreflective foils.

[0003] DE 196 39 615 C2 discloses a so-called reflector referencingsystem for surgical and medical instruments, which is formed from aradiation source and a reflector assembly having at least tworeflectors. That reflector system provides a marker system for effectingdetermination of the position of parts of the body or instruments.Surgical instruments for example can be fitted with a three-reflectoradaptor which delivers a reflection image which is characteristic ofthat instrument. In determining the position of parts of the body to betreated each landmark delivers an image which is characteristic only inrespect of itself, both in terms of diagnostic patient data acquisitionand also in terms of subsequent treatment monitoring. The reflectors arein the form of balls and are provided with a reflective coating. Suchballs produce a uniform reflection image, when considered from alldirections in space.

[0004] The passive marks which are fitted to devices and instruments inmedical engineering have to be frequently disinfected and sterilized.Sterilization of the medical instruments can be effected for example bymeans of gas sterilization or steam sterilization which can take up to afull working day, which means that with frequent use several sets ofinstruments have to be purchased. It is only in that way that it ispossible to guarantee that sterilized instruments are available at anytime.

[0005] Therefore, in order to achieve a reduction in complication andcosts, DE 196 39 615 C2, proposed that the very expensive marks areinterchangeably fitted to the medical devices and instruments. For thatpurpose the marks are fitted in the form of passive reflectors to anadaptor which in turn is releasably connected to the instrument. Thatnow admittedly means that the marks are interchangeable. The fundamentalproblem however is not eliminated. For, the known passive markswithstand the cleaning procedures only a few times and have to replacedby fresh marks after just a relatively few uses.

[0006] Based on the above-indicated state of the art, the object of thepresent invention is to produce a reflector system of the kind set forthin the opening part of this specification, involving the use of passivemarks which reflect over a wide angular range and which can withstand amultiplicity of the cleaning procedures which are possible hitherto, andcan be autoclaved virtually as often as may be desired. It will beappreciated in that respect that the medical demands also have to bemet.

[0007] In accordance with the invention that object is attained by thefeatures of claim 1. Advantageous configurations and developments of theinvention are described in the appendant claims.

[0008] The invention is based on the realization that the knownreflective coatings on balls or other geometrical mark bodies have onlya limited service life. That disadvantage is completely eliminated withthe present invention insofar as, in accordance with the invention, atransparent ball is used as a passive mark, which ball has a refractiveindex which can be selected in dependence on the desired angle of beamspread of a light beam and thus acts as a reflector in the UV-range, inthe visible range and in the infrared range (IR). In that respect thefront surface of the ball, on which the incident beams impinge, acts asa lens, with the focal point being on the rear surface of the ball.

[0009] The reflector system according to the invention makes use ofreflection at the transition of materials of different opticaldensities, which corresponds to a respective different refractive index.As there is no preferential direction in the case of the transparentball of such a nature, the ball provides for retroreflection in anyangle in space. As a result for example with a refractive index of1.9±0.1, for glass, light beams near the axis are reflected in the samedirection. In contrast the light beams which are remote from the axisare reflected at a given angle in space. By changing the refractiveindex it is possible to alter and appropriately maximize the lightoutput which is reflected at a given angle in space.

[0010] In an embodiment of the invention the surface of the transparentball can be entirely or partially part-transmissively mirror-coated ormetalized. The greatest reflected intensity of the light beams isachieved at a transmission of 67% or a reflection of 33%. As analternative thereto a predetermined angular segment of the reflector iscompletely metalized. As a result, a light beam is admittedly no longerreflected in all directions in space, but the reflected intensity issubstantially greater than in the case of part-transmissivemetalization.

[0011] In order to achieve more uniform distribution of the reflectedlight the metalized surfaces of the reflectors can preferably be of adiffusively reflective nature.

[0012] A particular configuration of the invention is achieved ifarranged between the ball and the reflection layer (metalization) is alayer of a transparent material with a different refractive index. Theangle of beam spread of the reflected light can then be influenced bythe refractive index of the ball, by the refractive index of theintermediate layer used and by the thickness of the intermediate layer.

[0013] Preferably the reflector system according to the invention isemployed in medical engineering for determining a position of a medicalinstrument or device or for determining parts of the body of patients.

[0014] An example of the invention is illustrated in the drawing inwhich:

[0015]FIG. 1 is a diagrammatic view of a medical instrument forneurosurgery with passive reflectors,

[0016]FIG. 2 shows a retroreflective ball with a refractive index ofabout 1.9, and

[0017]FIG. 3 shows a retroreflective ball with a partly metalizedsurface.

[0018] In quite general terms the man skilled in the art understands byreflection in physics the phenomenon that particles or waves, forexample sound or light, are thrown back at interfaces, such as forexample air and glass. With a very smooth interface, the law ofreflection then applies, which states that the angle of incidence of thelight beam is equal to the angle of reflection. The incident beam, thereflected beam and the normal of incidence lie in one plane. Regularreflection of that kind is experienced by a light beam at a mirror or ametalized surface of a body. If the surface roughnesses are greater thanthe wavelength of the beams, then so-called diffuse reflection occurs,in which the beams are reflected in all directions in space.

[0019] In most cases only a part of the incident radiation is reflectedwhile the other part is absorbed or refracted. At the transition from anoptically denser medium to a thinner medium, for example from glass toair or conversely from air to glass, only a part of the incidentradiation is reflected while the other part is refracted. In principlethe magnitude of reflection is dependent on the difference in refractiveindex of the two materials, in particular here glass. The light does notcontinue to go in a straight line but is refracted at the surface inaccordance with the optical law of refraction in dependence onrefractive indices.

[0020] The medical instrument shown in FIG. 1 represents a pointerinstrument for neurosurgical interventions. The medical instrument 1preferably operates in a cable-less manner and has a total of threepassive reflectors 3, 4 and 5. It will be appreciated that more or fewerthan three reflectors and other arrangements are also possible.

[0021] The reflectors 3, 4 and 5 are transparent balls which have aretroreflection action, as shown in FIGS. 2 and 3. Glass preferablyserves as the material for the ball. Electromagnetic radiation,generally a light beam, impinges on the surface 6 of the ball and isrefracted at the interface which exists between the outside air and theball consisting of the material glass. The air has a refractive index of1 while high-refraction glass has a refractive index of nearly 2. Thelight beam passes into the ball 3, 4 and 5 as indicated by the arrows 7,8 and 9. In this case the ball surface 6 acts like a convergent lens.The light beams 7 and 8 impinge on the surface of the ball and aredeflected at an angle A and A′ respectively and are passed onto thecommon end point 10 in the ball. The light beam 9 which is near the axisextends in a straight line after passing into the ball onto the endpoint 10. At that end point a part of the light beam is reflected and isreflected back again as indicated by the arrows 11, 12 and 13 and issuesfrom the surface of the ball at an angle. In the case of a mirrorsurface for the ball the angle of incidence is equal or almost equal tothe angle of reflection. The reflected light leaves the ball again, inwhich case the surface of the ball now again acts as a convergent lens.

[0022] In the example in FIG. 2 the ball (reflector 3, 4, 5) comprisesglass and has no coatings or mirror or metalization layers. This meansthat a part of the light beams which are incident into the ball issuefrom the ball at the end point 10. Another and generally smaller part ofthe light beams 7, 8 and 9 is reflected and issues from the ball againat the reflected locations 14, 15 and 16. Those reflected light beamswhich issue from the ball (reflector 3, 4, 5) are recorded by a receiversystem, for example a camera system, and suitably used. As the ball asshown in FIG. 2 is not metalized at any location, light beams canimpinge on the surface of the ball from all angles in space and beretroreflected. The angle of beam spread for the light beams, which isassociated with the transmitter, is relatively great. It is essentialwith this kind of reflector design that the light beams issuing afterreflection from the reflectors 3, 4 and 5 are so intensive that they arereliably received by the receiver.

[0023] In the embodiment shown in FIG. 3 a given angular segment 17 ofthe surface of the ball of the reflector 3, 4 and 5 is metalized. Thatmeasure provides that the light beams can only pass into the ball at agiven angle and be reflected there. Instead of the metalization in theangular segment 17, a diffusively scattering surface can also beenvisaged. Although with this embodiment light is no longer reflected inall directions in space, the advantage of this structure is the higherlevel of intensity of the issuing light beams. If the ball has adiffusively scattering surface, that affords more uniform distributionof the reflected light.

[0024] It will be appreciated that the non-metalized portion of thesurface of the ball can be dereflected.

[0025] The principle of retroreflection can moreover also be simulatedwith a lens and a concave mirror. If the focal length of the concavemirror is f, then the value 2f approximately applies in regard to thedistance from the lens to the concave mirror and 2f also applies inregard to the focal length of the lens. By varying the distance or byvarying the focal length of the lens it is possible to influence theangle of beam spread. A Fresnel lens can also be used in theabove-indicated example.

[0026] The reflectors 3, 4 and 5 are basically subject to the physicallaw of refraction. More specifically, if a light beam passes from anoptically thinner medium, in this case air, into an optically densermedium, in this case glass, the light beam is subjected to refraction,with the law of refraction known from physics applying.

[0027] It is expressly emphasized that these novel reflectors can beused generally and in many different ways in technology and the depicteduse in medical engineering only represents a preferred area of use.

1. A reflector system for determining position comprising at least onepassive reflector (3, 4, 5) being in the form of a transparent ball,characterized in that said reflector (3, 4, 5) being in the form of atransparent ball has a refractive index of 1.9±0.1 so that light beams(7, 8, 9) incident into one side of the ball and reflected on theopposite side of the ball are emitted (11, 12, 13) from said one side ofthe ball under an angle of beam spread with respect to said incidentlight beams.
 2. A reflector system according to claim 1, characterizedin that the surface of said transparent ball (3, 4, 5) is at leastpartially part-transmissively metalized.
 3. A reflector system accordingto claim 1 or 2, characterized in that an angular segment of the surfaceof said transparent ball (3, 4, 5) of predeterminable size is metalized.4. A reflector system according to claim 2 or 3, characterized in thatsaid metalized surface of said transparent ball (3, 4, 5) is of adiffusively reflective nature.
 5. A reflector system according to one ofthe preceding claims, characterized in that said transparent ball (3, 4,5) is formed from a material which is transparent only for a givenwavelength range.
 6. Use of a reflector system according to one of thepreceding claims in medical engineering for determining a position of amedical instrument or device or for determining parts of the body ofpatients.